Filters composed of an isotropic material are subject to premature clogging due to particulate collecting on the upstream surface of the filter, where fluid enters. Ideally filtered particles would be evenly distributed throughout the thickness of a filter so that longer filter life could be realized. The best way to achieve such a particulate distribution is to have porosity continuously decrease throughout the thickness of the filter in the direction of fluid flow through the filter.
One method of achieving a more uniform particulate distribution in a filter is disclosed in U.S. Pat. No. 6,926,828. The filter medium used in this invention is a flexible, isotropic, and porous material such as expanded foam, which is enclosed in a case body. The case body progressively compresses the filter medium along the fluid flow direction such that the pores in the filter material are progressively compressed tighter together, thereby capturing finer particles. This design requires an external structure such as a case body to support and compress the filter medium.
Another method of achieving a more uniform particulate distribution is to “intercalate” foam on a porous substrate as disclosed in U.S. Patent Pub. 2003/0084788 A1. This invention puts a polymeric or other type of expanding foam onto a porous substrate, then allowing the foam to expand. The expansion of the foam through the substrate and outside of the substrate produces distinct regions with different porosity. However, this does not produce a material with continuously and gradually decreasing pore size.
An attempt to produce a filter medium with varying pore sizes is disclosed in U.S. Pat. No. 6,387,141. This invention uses an isotropic nonwoven fibrous medium which is subjected to a liquid jetting. This compresses the fibers on the side facing the liquid jet, thereby reducing the porosity on the jetted side. Another embodiment of this invention is to mix fibers of different coarseness together to form at least two layers of different properties. The porosity is changed by changing the ratio of coarse fibers to fine fibers in the mixture forming each layer. The assembled layers are then liquid jetted on one side to produce intertwinements of the fibers that connect the layers.
Thus there remains a need to produce a filter that structurally supports itself, has a pore size that continuously and gradually decreases through the filtration medium, without relying on compression of the medium or mixing of different fibers to achieve a porosity gradient.